1. Field of the Invention
The present invention relates to a control unit for a stepping motor including an encoder, a method of controlling the stepping motor, and an optical apparatus.
2. Description of the Related Art
A stepping motor is characterized in that its position is easily controllable without a position detector because the stepping motor is rotatable every predetermined angle by switching the excitation to a coil. For this reason, an open-loop control is generally used to drive the stepping motor, which switches the excitation to the coil according to a predetermined time interval. However, disadvantageously, when it is driven at a high velocity or bears a heavy load, a rotor may fail to respond to the excitation switching to the coil, thereby it is likely to step out.
Japanese Patent Laid-Open No. (“JP”) 10-150798 proposes a method for switching an open-loop control and a feedback control (closed-loop control) according to a movement amount, by including an encoder in a stepping motor. JP 2002-148502 proposes a method for switching a motor exciting method in an interchangeable lens type image pickup apparatus according to a type of the image pickup apparatus to be attached. In addition, the micro step driving that applies a sine wave to a motor driving waveform is a known driving method for reducing driving noises and for precisely positioning the stepping motor.
It is difficult to feedback-control a stepping motor with an encoder in the entire velocity range from the low velocity to the high velocity while the positioning accuracy is maintained, and driving vibrations and noises are reduced. Moreover, since a necessary motor characteristic is different in an image pickup apparatus according to a capture mode such as a still image capture mode and a motion image capture mode, an appropriate motor control is necessary for each capture mode. Furthermore, the necessary motor characteristic is different according to a focusing type, such as autofocusing and manual focusing.